This invention relates to a cylinder liner of a hypereutectic aluminum/silicon alloy for casting into a reciprocating piston engine and a process for producing such a cylinder liner.
EP 367,229 A1 shows that a cylinder liner which is produced from metal powder and mixed-in graphite particles (0.5 to 3%; grain diameter at most 10 .mu.m or less, measured in a plane measured transversely to the cylinder axis) and hard material particles without sharp edges (3 to 5%; grain diameter at most 30 .mu.m, average 10 .mu.m or less), in particular alumina, is known. The metal powder is initially produced on its own, that is to say without admixed particles other than metals, by air atomization of a hypereutectic aluminum/silicon alloy having the following composition--the remainder being aluminum--(in percent by weight, relative to the total metal content of the alloy, that is to say without the hard material particles and graphite fractions not present in the melt):
______________________________________ Silicon 16 to 18%, Iron 4 to 6%, Copper 2 to 4%, Magnesium 0.5 to 2% and Manganese 0.1 to 0.8%. ______________________________________
The metal powder is mixed with non-metallic particles and this powder mixture is pressed at about 2,000 bar to give a preferably tubular body. This powder metallurgically produced blank is inserted into a piece of soft-aluminum tube, corresponding to the form, and the two-layer tube obtained in this way is sintered and formed, preferably at elevated temperatures, to give a tubular blank from which the individual cylinder liners can be produced. The embedded hard material particles are intended to confer good wear resistance onto the cylinder liner, whereas the graphite particles serve as a dry lubricant. To avoid oxidation of the graphite particles, the hot extrusion should be carried out with exclusion of oxygen. There is also the risk that, at high processing temperatures, the graphite reacts with the silicon and superficially hard SiC is formed, whereby the dry lubrication property of the embedded graphite particles is impaired. Since the powder mixture is always more or less complete, it can never be entirely ruled out that locally more or less extensive fluctuations in the concentration of hard material particles and/or graphite particles occur on the surface of the workpiece. Due to the embedded hard material particles, the hot-pressing mould wears out relatively rapidly, since the hard material particles still have, in spite of their rounded edges, a powerfully abrasive action; with reasonable effort, it is in any case possible only to round the edges partially on the particles formed by crushing comminution. The subsequent mechanical treatment of the running surface of the cylinder liner also entails high tool wear and thus high tool costs. The hard material particles exposed in the running surface have sharp edged boundaries after the surface machining and subject the piston skirt and the piston rings to relatively extensive wear, so that these must be produced from a wear-resistant material and/or must be provided with an appropriately wear-resistant coating. The known cylinder liner altogether is not only relatively expensive due to the starting materials with several separate components, but the high tool costs in connection with the plastic and metal-removing machining greatly increase the cost per piece. Apart from this, the type of manufacture of the known cylinder liner from a heterogeneous powder mixture involves the risk of inhomogeneities which, under some circumstances, cause a functional impairment, that is to say rejects, but in any case require expensive quality monitoring. Furthermore, it presupposes piston designs which are complex in engine operation and which altogether make the reciprocating piston engine more expensive.
U.S. Pat. No. 4,938,810, which likewise shows that a powder-metallurgically produced cylinder liner is known, should also be mentioned. In this case, a large number of alloy examples are listed, and measurement data and operating data of the cylinder liners produced with these are also given. The silicon contents of the examples given are in the range from 17.2 to 23.6%, even though a more comprehensive range from 10 to 30%, which extends down into the hypoeutectic range, is taught. At least one of the metals, namely nickel, iron or manganese, should likewise be present in the alloy, at least in an amount of 5% or (iron) at least in an amount of 3%. As a representative, only one alloy composition in % by weight will be mentioned here; zinc and manganese contents are not given, which leads to the conclusion that these metals, apart from traces, is not present:
Silicon: 22.8%, PA0 Copper: 3.1%, PA0 Magnesium: 1.3%, PA0 Iron: 0.5% and PA0 Nickel: 8.0%, PA0 the remainder being aluminum. PA0 Silicon: 25%, PA0 Copper: 4.3%, PA0 Magnesium: 0.65% and PA0 Iron: 0.8%, PA0 the remainder being aluminum.
The nickel content in the alloy example given is very high. A blank for a cylinder liner is hot-extruded from the powder mixture.
U.S. Pat. No. 4,155,756, deals with the same topic. In this case, inter alia, the following composition of a powder-metallurgically produced cylinder liner is given as one example of several: